1. Field of the Invention
The present invention relates to a linear motor and side mounted outboard carriage combination and, more particularly, to such a linear motor and carriage in which the center of gravity of the carriage and motor armature is aligned with the center of force of the linear motor.
2. Description of the Prior Art
While linear electric motors have many uses in industry, the present invention will be described with reference to a linear motor and carriage for use in a magnetic disc storage system for moving read-write transducers over the surface of moving data media discs. Such use of a linear motor calls for extremely high precision of movement.
Magnetic disc storage systems of the type in which the present linear motor may be used are widely used to provide large volumes of relatively low-cost computer-accessible memory or storage. A typical disc storage device includes a number of discs coated with a suitable magnetic material mounted for rotation on a common spindle and a set of transducer heads carried in pairs on elongate supports for insertion between adjacent discs, the heads of each pair facing in opposite directions to engage opposite faces of adjacent discs. The support structure is coupled to the linear motor, the linear motor typically including a coil mounted within a magnetic field for linear movement and oriented relative to the discs to move the heads radially over the disc surfaces to thereby enable the heads to be positioned over any annular track on the surfaces. In normal operation, the linear motor, in response to control signals from the computer, positions the transducer heads radially for recording data signals on or retrieving data signals from a preselected one of a set of concentric recording tracks on the discs.
A widely used computer motor is open at one end and a helical armature coil supported by a matching cylindrical tube projects into the open end of the motor. The coil is adhered to the end of the tube and the tube is mounted on a carriage that moves back and forth under influence of the motor coil. The equipment to be moved is mounted on the carriage. The carriage moves on accurately ground ways so that the equipment (read-write transducers) secured to the carriage is moved with great accuracy.
It has been generally believed that the dynamic accuracy necessary for movement of read-write transducers along an exact axis requires that the carriage be coaxial with the linear motor and that the mounting for the transducers also be coaxial. It has been generally believed that the non-coaxial transfer of motion from the armature coil would result in torques and moments that would so distort the carriage and its components that the required accuracy could not be achieved.
Unfortunately, when mounting the carriage coaxially with the linear motor, considerable length is required to accommodate the heads, coil and magnetic structure. With the present emphasis on providing magnetic storage systems in as small a package as possible, too much longitudinal space is required for a motor/carriage coaxial arrangement.
As a space saving innovation, it has been proposed to provide a carriage mounted on the side of a linear electric motor for motion parallel to the armature axis. A typical such combined linear motor and side mounted carriage combination is disclosed and claimed in U.S. Pat. No. 3,656,015 to Donald E. Gillum and entitled "Combined Linear Motor and Carriage". By mounting the carriage off to one side, the magnetic structure need not be in series with the heads and the carriage.
Experience has shown that serious problems do indeed arise from this type of structure because the center of gravity of the moving mass does not line up with the center of force provided by the motor. Structural resonance has proved to be a serious problem with this configuration since the offset force excites lateral vibration modes not seen in a coaxial structure. The only think that resists the lateral force is the bearing preload and, under certain circumstances, this can be overcome, knocking the carriage bearings off their ways. This often occurs in a runaway situation. That is, one problem with linear motors and servo loops in general is that they can get lost. In other words, the computer could signal the linear motor to move the transducer head in one direction, searching for a track which the head has in fact already passed. Often, the head continues accelerating until it reaches a mechanical stop at the end of its travel. The impact of hitting the mechanical stop with the center of gravity of the moving mass misaligned with the center of force can and does knock a bearing off its track because the force overcomes the bearing preload.